As it is known, many food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may be defined by a layer of fibrous material, e.g. paper, or mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material may also comprise a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which the tube is formed continuously from the web-fed packaging material. More specifically, the web of packaging material is unwound off a reel and fed through a station for applying a sealing strip of heat-seal plastic material, and through an aseptic chamber on the packaging machine, where it is sterilized, e.g. by applying a sterilizing agent such as hydrogen peroxide, which is subsequently evaporated by heating.
The web of packaging material is then fed through a number of forming assemblies which interact with the packaging material to fold it gradually from strip form into a tube shape.
More specifically, a first portion of the sealing strip is applied to a first longitudinal edge of the packaging material, on the face of the material eventually forming the inside of the packages; and a second portion of the sealing strip projects from the first longitudinal edge.
The forming assemblies are arranged in succession, and comprise respective roller folding members defining a number of packaging material passages varying gradually in cross-section from a C shape to a substantially circular shape.
On interacting with the folding members, the second longitudinal edge is laid on the outside of the first longitudinal edge with respect to the axis of the tube being formed. More specifically, the sealing strip is located entirely inside the tube, and the face of the second longitudinal edge facing the axis of the tube is superimposed partly on the second portion of the sealing strip, and partly on the face of the first longitudinal edge located on the opposite side to the first portion of the sealing strip.
Packaging machines of the above type are known in which the first and second longitudinal edges are heat sealed within the aseptic chamber to form a longitudinal seal along the tube, which is then filled with the sterilized or pasteurized food product.
Furthermore, the packaging machines of the above type comprise a forming unit in which the tube and is sealed and cut along equally spaced cross sections to form pillow packs.
The forming unit comprise two or more jaws which cyclically interact with the tube to seal it.
The pillow packs are then folded mechanically to form respective packages at a folding unit, which is arranged downstream of the movable components of the forming unit.
In detail, the forming unit is arranged downstream of the aseptic chamber, with reference to the advancing direction of the tube.
The above described packaging machine comprises a plurality of branches which output relative flows of hot sterile air, e.g. at a temperature ranging between 5 to 280° C., inside the aseptic chamber.
In particular, a first branch comprises a plurality of nozzles which output a first hot sterile air flow inside the aseptic chamber in order to keep it at the given value of temperature and pressure greater than the environment pressure.
A second branch comprises a nozzle which outputs a second hot sterile air flow onto the superimposed longitudinal edges, so as to form the longitudinal seal along the tube.
A third branch comprises a nozzle which is arranged downstream of the second branch, proceeding according to the advancing direction of the tube.
The third branch is activated only when the operation of the packaging machine starts again after an interruption.
In case of interruption, the portion of packaging material facing the nozzle of the second branch cools down, after having been previously heated.
The re-start of the packaging machine brings that portion in front of the nozzle of the third branch. At this stage, the nozzle of third branch is operated to output a third hot sterile air flow towards the packaging material. That third hot sterile air flow heats again this portion of the packaging material and ensure that the complete formation of the longitudinal seal.
Finally, a fourth branch comprises a nozzle which outputs a hot sterile air flow onto the web packaging material upstream of the aseptic chamber and before the packaging material is formed into a tube shape, in order to remove, by heating, the residual of hydrogen peroxide from the packaging material.
The above identified hot sterile air flows are regulated by respective control valves.
In particular, control valves are known which substantially comprise:                an outer body which defines an inlet opening and outlet opening for the sterile air flow; and        a disk or ball shaped shutter housed inside the outer body.        
The shutter can be rotated between:                a fully open position in which it allows the hot sterile air to flow from the inlet opening to the outlet opening of the body; and        a fully closed position in which it prevents the hot sterile air from flowing between the inlet opening and the outlet opening.        
Due to the shape and construction of the shutter, the known valve has a poor capability of smoothly modulating the flow of hot sterile air flow.
In particular, with the known valve, the amount of sterile air flow suddenly grows from zero to the maximum value, when the shutter rotates from the fully closed position to the fully open position.
In other words, the plot of sterile air flow versus a rotation angle of the shutter is steep, is highly non-linear and reaches the maximum value after a small rotation angle of the shutter.
As a result, the hot sterile air flow is not precisely controllable.
A need is felt within the industry to obtain a variation as linear as possible of the sterile air flow in relation to the rotation angle for a wide range of rotation angle of the shutter, with a limited number of components and without affecting the possibility of preserving the sterility of the shutter.